I haven't done it yet, but will add a footpump on my boat. You probably need to insert a "T" in the line between the tank and electric pump
. Make sure you also put a check valve in so your electric pump
doesn't suck air in through the foot pump and end up cavitating.
I suspect the reason they don't want you to mount the footpump after the electric
pump is that the footpump is not tight. It will leak when under pressure, causing the electric
pump to start and soon you'll have empty tanks